Pakistan Vet. J., 24(4): 2004
EFFECT OF DECREASING DIETARY PROTEIN LEVELS WITH OPTIMAL
AMINO ACIDS PROFILE ON THE PERFORMANCE OF BROILERS
Z. Kamran, M. Aslam Mirza, Ahsan-ul-Haq1 and S. Mahmood1
Institute of Animal Nutrition and Feed Technology, 1Department of Poultry Husbandry,
University of Agriculture, Faisalabad, Pakistan
A six-week trial was conducted to study the effect of decreasing dietary crude protein (CP) level on the
performance of broilers in hot climatic conditions. Four experimental rations having CP 23 (control group),
22, 21 and 20%, with optimal amino acid balance were prepared. All the four rations were isocaloric
having ME 3200 kcal/kg with Energy: Protein (E:P) 139.0, 146.5, 152.4 and 160 in diets A, B, C and D
respectively. One hundred and twenty day-old chicks were randomly distributed into 12 experimental units,
each having 10 chicks. Rations were randomly allotted to experimental units such that each unit received
three replicates. The experimental diets were fed to birds from day 1st to 42nd. Performance of birds was
monitored in terms of weight gain, feed consumption and feed conversion ratio (FCR). At the end of
experiment, two birds per each replicate were randomly selected and slaughtered to record the data on
carcass yield, breast meat yield, abdominal fat and composition of breast meat. Results of the trial
suggested that weight gain was significantly (P<0.01) increased in birds on diets with CP 20 and 21%. Feed
consumption and FCR remained un-changed for all the treatment groups. Eviscerated carcass yield was
significantly (P<0.05) higher for the group fed on diet with 20% CP. Breast meat yield, abdominal fat and
composition of breast meat also remained un-changed. Economic evaluation of the trial revealed that
decreasing CP levels from 23 to 20% resulted in reduced feed cost per kg of live weight gain, which clearly
indicated that this approach was useful especially in severe summer conditions. The overall picture of the
study suggests that dietary protein level of broilers could be reduced from 23 to 20%, with beneficial
effects on growth performance and carcass characteristics and increased economic returns in hot
environmental conditions, provided that levels of essential amino acids are closely looked after.
Key words: Dietary protein, amino acids, broilers.
A major concern of modern poultry enterprise is to
reduce feed cost for optimal economic returns because
feed constitutes approximately 70% of the total
production cost. One way to reducing the feed cost is
through improvement in the feed efficiency of birds.
While formulating a broiler’s diet, the main emphasis is
placed on the crude protein (CP), because it is one of
the major cost components of the poultry diets. Broilers
have high dietary CP needs. Dietary protein level,
therefore, has major effect on growth performance and
overall cost of finished product. Dietary CP level could
possibly be reduced if there were adequate of the
minimum levels of amino acids needed to support
growth and muscle of broilers (Firman and Boling,
1998). Broiler chicks fed diets marginal in protein but
fortified with methionine and lysine have been reported
to perform well as those fed a diet higher in protein
(Jensen and Colnago, 1991).
Birds utilize excess dietary amino acids
inefficiently because the amino acids surplus to the
birds’ requirements are deaminated and nitrogen is
excreted as uric acid. On the other hand, birds fed on
diets marginal in amino acids will over consume to
meet their requirements for gain, thus resulting in
increased carcass fat contents with reduced feed
efficiency (Thomas et al., 1978). If the amino acid
pattern provided in the diet exactly matches the birds’
amino acids requirements, excesses can be avoided.
Consequently, protein accretion can occur with
maximum efficiency (Chung and Baker, 1992).
Reduction in CP also reduces the nitrogen (N) level in
excreta and litter, thereby decreasing disposal problems
(Moran et al., 1992). Feeding low CP diet has also been
reported to improve the performance of birds in hot
temperatures (Thim et al., 1997).
The present trial was envisaged to determine the
effect of decreasing dietary CP content (from 23 to
20%) while maintaining optimal essential amino acids
Pakistan Vet. J., 24(4): 2004
profile on the performance of broilers in hot climatic
MATERIALS AND METHODS
One hundred and twenty day-old broiler chicks
(Hubbard) of mixed sex were wing banded and
randomly divided into twelve experimental units
(replicates) of 10 chicks each. Four experimental broiler
rations (A, B, C, D) were formulated (Table 1). Ration
A served as control with 23% CP. In rations B, C and
D, the CP was reduced to 22, 21 and 20%,
respectively. All four rations were isocaloric (ME =
3200 Kcal/kg) with Energy: Protein (E:P) 139.0, 146.5,
152.4 and 160 in diets A, B, C and D respectively.
Lysine, which was maintained at 1.1% in each ration,
was chosen as reference amino acid to which all other
essential amino acids were ratioed in ideal acid pattern
Each of four rations was allotted randomly to 3
replicates. The experimental diets were fed to birds
from days 1 to 42. Performance of birds was monitored
in terms of weight gain, feed consumption and feed
conversion ratio. At the end of experiment, two birds
per replicate were randomly selected and slaughtered to
record the data on carcass yield, breast meat yield,
abdominal fat and composition of breast meat. The
economics of all the rations was also calculated.
The data thus obtained on various parameters were
subjected to statistical analysis according to completely
randomized design using analysis of variance
technique. Duncan’s Multiple Range test was applied to
compare the significance of differences between the
means (Steel and Torrie, 1980).
RESULTS AND DISCUSSION
Dilution of the CP content of the diet, while
maintaining essential amino acids as per NRC (1994) in
hot climatic condition, resulted in a significant (P<0.01)
increase in weight gain in chicks of groups C and D fed
on experimental diets with 21 and 20% CP as compared
to control (group A) with 23% CP and group B with
22% CP (Table 2). Similar results were reported by
Thim et al. (1997) when dietary protein level was
reduced to 20% at a temperature higher than 26.7ºC.
An increase in feed intake was also noted in
response to protein dilution and energy:protein
widening which was non-significant. As the calorific
value of all the experimental rations was the same and
even the diet with the lowest protein level (i.e. 20%)
supplied sufficient amount of essential amino acids
required for the growth of birds, it was natural that the
feed consumption of the birds remained the same on
different experimental diets. The results of the present
trial on feed intake are in accordance with the feedings
of Han et al. (1992), Bartov and Plavnik (1998) and Hai
and Blaha (1998), who found no difference in feed
intake of broiers when dietary CP contents were
decreased from 23 to 20%. The results are however, not
supported by the findings of Kidd et al. (2001),
Bregendahl et al. (2002) and Ferguson et al. (1998),
who found significant increase in feed intake by broiler
chicks fed on diet with CP 20% and supplemented with
amino acids as against those fed a 23% CP diet.
Feed conversion ratio (FCR) was slightly improved
by decreasing dietary CP level. The difference was,
however, non-significant. The results are in accordance
with the findings of Han et al. (1992), Bartov and
Plavnik (1998) and Hai and Blaha (1998), who found
no difference in FCR of broilers when the CP content of
the diet was decreased from 23 to 20%. The results of
the present trial are not supported by the findings of
Kidd et al. (2001), Bregendahl et al. (2002) and
Ferguson et al. (1998), who noted significant increase
in FCR of broiler chicks fed diets with 20% CP
supplemented with essential amino acids (EAA)
compared to those fed a diet with 23% CP.
Dilution of the CP content of the diet resulted in
significant (P<0.05) increase in eviscerated carcass
weight/yield (Table 2). The improvement in weight
gain and carcass yield with the use of low CP diet
(supplemented with EAA) could be due to reduced heat
increment, which was associated with the metabolism
of excess protein. Reduced heat increment led to
reduced heat stress and, therefore, improved feed intake
and weight gain. Kidd et al. (2001) and Bartov and
Plavnik (1998) also reported increased carcass yield
when a low protein diet (19%) was fed as compared to
high protein (22.5%) diet. In contrast, Bregendahl et al.
(2002) reported that chicks fed low CP diet (20%)
supplemented with essential amino acids gained less
weight as against the control diet (23% CP). Breast
meat yield, abdominal fat and CP and edible energy
content of breast meat remained unaffected in response
to dietary protein dilution.
Overall, the results of the present trial indicated
that decreasing dietary CP level associated with optimal
essential amino acids did not significantly affect the
carcass characteristics. It was also obvious from these
results that decrease in CP was not associated with over
consumption of feed, hence there was not any
appreciable increase in abdominal fat contents. The
results corresponded with those of Han et al. (1992).
Economic evaluation of the trial revealed that
decreasing CP levels from 23 to 20% resulted in
reduced feed cost per kg of live weight gain which
clearly indicated that this approach was useful
Pakistan Vet. J., 24(4): 2004
Table 1: Percent ingredient composition of experimental rations
A C D
Cotton seed meal
Corn gluten (60%)
Calculated nutrient and amino acid composition
Table 2: The effect of decreasing protein level with optimal amino acids profile on the growth
performance and carcass characteristics of broilers during 0-6 weeks
Average weight gain (g/chick) 1408b
Average feed intake (g/chick) 2970a
Feed conversion ratio 2.11a
Carcass yield (%) 55.13c
Breast meat yield (%) 23.20a
Abdominal fat 3.03a
CP content of breast meat 70.79a
EE content of breast meat 7.12a
Cost/kg live weight gain (Rs) 24.69
Mean values within rows showing different superscripts differ significantly (P<0.05)
Dietary CP levels (%)
Pakistan Vet. J., 24(4): 2004
especially in severe summer conditions. The overall
picture of the study suggests that dietary protein level
of broilers could be reduced from 23 to 20%, with
beneficial effects on growth performance and carcass
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environmental conditions, provided levels of essential
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